We have previously found that at the time of fertilization, a single sea urchin spermatozoon contains at least 2 X 10(-18) moles of inositol trisphosphate, a second messenger known to cause release of calcium from intracellular organelles. We have also calculated the relative activation efficiencies of direct insertion by the spermatozoon and insertion by injection, and found that direct insertion by the spermatozoon is about three times more efficient than insertion by injection. Since the quantity of inositol trisphosphate required to initiate activation of sea urchin eggs by injection is known to be about 3 X 10(-18) moles, it is likely that a single spermatozoon can insert enough inositol trisphosphate into the egg to initiate activation, and thus, that inositol trisphosphate may act as a primary messenger in the fertilization process. We are planning to test this hypothesis by loading liposomes with appropriate amounts of inositol trisphosphate, and observe whether the liposomes can activate eggs.